1. Field of the Invention
The disclosure relates generally to devices for carrier recovery and related methods, and, more particularly to devices for carrier recovery and related methods for use in DMB-T (Digital Multimedia Broadcast-Terrestrial) systems.
2. Description of the Related Art
Currently, DMB-T defines a digital data transmission standard in China. DMB-T adopts modulation technology of TDS-OFDM (Time Domain Synchronous-Orthogonal Frequency Division Multiplexing). The DMB-T system has a layered-structure of frames. A frame group is a group of frames, wherein the first frame is the frame group header. A super frame defines a group of frame groups. The top layer of the frame structure is called a CDF (Calendar Day Frame). The physical channel is synchronized with absolute time. Signal frames are basic units in DMB-T.
The baseband signal of a frame body is an OFDM block. An OFDM block can be further divided into a guard interval and an IDFT (Inverse Discrete Fourier Transform) block. For TDS-OFDM, PN (Pseudo Noise) synchronization sequence is used as the guard interval for flame synchronization and OFDM, and the frame body is used as the DFT block. Since the PN sequence and the DFT block are OTDM (Orthogonal Time Division Multiplexing), and the PN sequence is a known sequence for the receiving end, and the PN sequence and the DFT block can be separated at the receiving end.
For digital communication or broadcast systems with synchronous transmission, frequency deviation may occur in receiving signals due to Doppler effect and local carrier errors, thus reducing the performance of the communication systems, and causing unexpected and dangerous results. Therefore, for OFDM signals composed of many orthogonal carriers, the bandwidth of sub-channels is much smaller than the entire bandwidth. The frequency offset of carriers will effect the orthogonal of sub-carriers and cause inter-carrier interference. A small frequency offset will cause a serious signal-to-interference ratio.
Conventional transmission methods have the following drawbacks.
(1) The results of CFE (Coarse Frequency Estimation) and de-correlation AFC (Auto Frequency Control) have large errors. A CFE estimator comprises a sequence of a square circuit, a delay circuit and a conjugate operation circuit series-connected, and a sequence of a multiplication circuit, an accumulator and a modulation circuit series-connected. The two inputs of the multiplication circuit are respectively connected with the outputs of the square circuit and a multiplication circuit series-connected. The square circuit has an input to receive a PN sequence. The delay length of the delay circuit is I1. The accumulation length of the accumulator is the length K of the PN sequence. The inputs of the modulation circuit are signals output from the accumulator. The modulation circuit performs modulation, and multiplies a coefficient of ½*I1 to the modulated signal. The output signal of the modulation circuit is the CFE of a current frame. A de-correlation AFC estimator comprises a sequence of a square circuit, a delay circuit and a conjugate operation circuit series-connected, and a sequence of a multiplication circuit, an accumulator and a modulation circuit series-connected. The square circuit has an input to receive a PN sequence. The delay length of the delay circuit is I2. The two inputs of the multiplication circuit are respectively connected with the outputs of the square circuit and the conjugate operation circuit. The accumulation length of the accumulator is the length K of the PN sequence. The inputs of the modulation circuit are signals output from the accumulator. The modulation circuit performs modulation, and multiplies a coefficient of ½*I2 to the modulated signal. The output signal of the modulation circuit is the de-correlation AFC of the current frame.
(2) In simulation, when the local PN code and the received baseband signals are correlated for frame synchronization, the frequency shift between the transmitting end and the receiving end exceeds 20k. No obvious peak can be located in a frame header signal PN420 with a frame header mode length of 420 symbols for frame synchronization. When the frequency shift exceeds 10k, frame synchronization will fail for a frame header signal PN945 with a frame header mode length of 945 symbols. It is difficult to distinguish the frame header and the frame body, such that no appropriate PN sequence can be found for CFE and de-correlation AFC.
(3) Even if frame synchronization can be accurately found, the results of CFE and de-correlation AFC may also have large errors if multi-path effect exists or the signal-to-noise ratio is low, thus causing subsequent time synchronizations and the entire reception process to fail.
Therefore, it is an important issue to correct frequency shifts in communication broadcasts.